Mat for cardio on carpet?

I love doing Turbo Jam and Zumba DVDs for my workouts, but the only places in my home where I have room to do them is on carpet. This poses obvious difficulties, as athletic shoes don't pivot or move well on carpet. It's killing me knees! And I can't do it without shoes on as I need the support.

Has anyone found a mat that works well for doing cardio on carpet? I have been looking online and I can't seem to find many tried-and-true options.

I have actually thought about getting one of those clear plastic mats that goes under a rolling computer chair and has little spikes to keep it in place, but I don't know it that's a good or bad idea.

Any recommendations?